3.1.1. Resistive switching of PCMO film with Pt electrodes. Chen et al. [106] reported the resistive switching properties of PCMO thin films with Pt electrodes. A schematic structure of the Pr0.7Ca0.3MnO3 (PCMO) devices is shows in Fig. 26(a). A 30 nm thick sputtered deposited PCMO layer is sandwiched between a Pt bottom electrode (BE) and a Pt top electrode (TE). The device was fabricated on a patterned TiN/Ti substrate.
Fig. 26(b) shows a cross-sectional transmission electron microscopy (TEM) image of a Pt/PCMO/Pt heterostructure deposited on SiO2/ Si substrate, where the columnar PCMO
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Figure 27. (a) DC I–V hysteresis sweep of a 500 nm× 1000 nm PCMO device and (b) EPIR performance of the 500 nm× 1000 nm device [after ref. 106].
grains are clearly observed. The high-resolution TEM (HRTEM) image in Fig. 26(c) reveals a well-defined and unreacted oxide/metal interface between the PCMO layer and the Pt BE.
Bipolar resistance switching phenomena was observed from the I–V hysteresis curve in the PCMO devices as shown in Fig. 27(a). The device switches from the HRS to LRS (set) at+1.6 Volt, while the device switches from the LRS to HRS (reset) at −2.2 Volt. It is observed that the dc voltages required switching the device and it is almost independent on the device size. The EPIR bipolar switching was also observed for these PCMO devices. Fig. 27(b) shows the EPIR performance of the 500 nm× 1000 nm PCMO device for 1.5×103 programming cycles. To set the device, 2.5 Volt 50-μs pulse-width voltage pulse was applied to the TE and, while a negative electric pulse of −2.5 Volt with a 10-ns pulse-width, was used to reset the device. Rhigh and Rlow denote the resis-tance of the PCMO device at the HRS and LRS, respectively. The EPIR ratio is defined as (Rhigh− Rlow)/Rlow, which is close to 200% as shown in Fig. 27(b). The average val-ues of the Rhigh’s and Rlow’s are 17.16 and 5.69 K, respectively and it’s almost stable (upto more than 1.5×103cycles) during all of the programming cycles, shows outstand-ing cycle-to-cycle stability, i.e., the standard deviations of all samples of the Rhigh and Rlow at all cycles are only 81 and 44 , respectively. The device showed good retention after 24 hour at 150◦C, no resistance degradations between two resistance states were observed.
3.1.2. Resistive switching in PCMO films with multilayer graphene electrode. Lee et al.
reported the resistive switching characteristics in PCMO devices using multilayer graphene (MLG) as a top electrode (TE) [108]. Interfacial-reaction-type resistive switching was ob-served upon introducing MLG as a conducting electrode to electrochemically functionalize graphene at the MLG/PCMO interface. A 60-nm thick PCMO film was deposited by rf sputtering on Pt/SiO2/Si substrate. Initially the MLG films were fabricated on nickel films by chemical vapor deposition (CVD) [142]. Finally the MLG films were detached from the nickel layers and transferred to the PCMO/Pt/SiO2/Si substrate. A structure with Pt TE was also fabricated as a reference for this study.
The good quality of the MLG films was confirmed from the Raman spectroscopy. Sheet resistances of MLG films were in the range of 700±100 /cm2. The typical current-voltage bipolar switching characteristic of the MLG/PCMO/Pt device is shown in Fig. 28(a). A bias was applied to top electrode (MLG), whereas bottom electrode (Pt) was grounded,
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electrode. (b) Pulse endurance and (c) retention characteristics of MLG/PCMO/Pt, indicating stable HRS and LRS for up to 104s at 85◦C [after ref. 108].
as shown in the left inset of Fig. 28(a). On increasing the applied positive voltage from 0 to 5 V, the device state changed from the LRS to the HRS, whereas on increasing the applied negative voltage from 0 to−5 Volt, LRS was obtained again. No forming process required for this device [106]. No resistive switching observed for the device having Pt TE of this PCMO based device structure, as shown in the right inset of Fig. 28(a). The device shows a negligible degradation of the HRS and LRS upto 103cycles, confirmed from the pulse endurance characteristics as shown in Fig. 28(b). Fig. 28(c) shows the retention characteristics of this device at 85◦C. Stable HRS and LRS with high resistance ratio were observed upto 104seconds.
The switching mechanisms of the MLG/PCMO/Pt structures are shown in Fig. 29.
When a positive bias is applied, oxygen ions in PCMO are incorporated into the MLG layer and an oxygenated graphene layer is formed at the MLG/PCMO interface, as illustrated in Fig. 29(a). Therefore, the total resistance of the layer is increases and switching from LRS to HRS is achieved. The device acts like GO/PCMO device structure as discussed in the previous section. Conversely, when a negative bias is applied, oxygen ions are extracted from the oxygenated graphene, resulting in the almost dissolution of the oxygenated graphene layer at the MLG/PCMO interface. As a result, the conductivity of oxygenated graphene increases and the device switches to LRS, as shown in Fig. 29(b). It is important to note that for this structure the device switched from the LRS to HRS and then HRS to LRS repeatedly.
Figure 29. Schematic illustrations of resistive switching behavior of MLG/PCMO/Pt device; (a) HRS and (b) LRS [after ref. 108].
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Mostly PCMO forms Ohmic contact with Pt [109, 143]. The structure with PCMO1 active layer shows lower initial resistance (100–800 ), whereas the structure with PCMO2 layer shows relatively higher initial resistance (1–200 M). The two structures show the nonlinear and almost symmetric current–voltage (I–V) curves, as shown in Fig. 30(a), with no evidence of hysteresis loops. In compare, the Pt/PCMO2/PCMO1/Pt structures showed bipolar resistive switching behavior, as shown in Fig. 30(a). It required a forming process, of about 4.5 Volt forming voltage, (10 mA current compliance) to switch the device to HRS state, as shown in the inset of Fig. 30(b). After the initial forming process the memory device switched to LRS. The device is reset (switched from LRS to HRS) by sweeping the voltage to positive direction, at about 0.9 Volt. Subsequently, the set process by applying−3 Volt reset voltage, and the nonvolatile switching was observed. Area dependence of the two resistance states is shown in Fig. 30(c). Resistance at LRS is almost independent with the electrode size, whereas the resistance at HRS decreases with electrode area. Which is indicates that the formation and rupture of the localized conductive filaments are responsible for such charac-teristics of this bilayer structures. I–V curve of LRS exhibits Ohmic behavior with a slope of 1 indicates the formation of conducting filaments during set process. However, the I–V curve of HRS in the low voltage shows some fluctuation. At low field region the Ohmic con-duction (I–V) behavior and in the higher voltage the trap free SCLC concon-duction mechanism is observed.
3.2.2. Doped PZT/PCMO heterostructure resistive memory. Bourim et al. [110], reported the resistive switching characteristics of an all perovskite heterostructure composed of an active Al-Nb co-doped Pb(Zr0.58Ti0.42)O3(PZT) ferroelectric thin film and a semiconduct-ing Pr0.7Ca0.3MnO3 (PCMO) layer, both sandwiched between the Pt electrodes. The Al and Nb doped PZT and PCMO thin films were grown by PLD. Typical thickness of the deposited crystalline PZT film was∼15 nm while that for polycrystalline PCMO films was varied from 20 to 240 nm.
Figure 31 shows the typical DC I–V characteristics of the fabricated Pt/PZT/
PCMO(60 nm)/Pt device. After first voltage sweep (0→+2 Volt→0→−2 Volt→0) re-ferred to as electroforming in which the current underwent an sudden decrease in the positive voltage region (dashed curve in Fig. 31) that is associated to a strong switching polarization current (charging-discharging at the capacitor electrodes) resulting from the first reorientation and reversal of ferroelectric domains in the PZT film. An increase of voltage sweeping bias amplitude to Vmax = ±4 Volt, the reversible counter clockwise I–V hysteresis loops evolved in both positive and negative voltage regions as shown by arrows in the I–V curves of Fig. 31. The switching curves representing up to several cycles exhibit a near-perfect stability and reproducibility as well as highly symmetrical hysteresis curves. To realized the transport properties after the application of a voltage
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Figure 30. Typical I–V characteristics of (a) Pt/PCMO1/Pt and Pt/PCMO2/Pt, (b) Pt/PCMO2/PCMO1/Pt structures in linear scale. The inset of (b) shows the forming process and the unipolar switching during positive voltage sweep. (c) The change of resistance as the device area changes, giving an indication of filament-type switching in Pt/PCMO2/PCMO1/Pt memory devices.
The inset of (c) shows the typical I–V characteristics and their fit results shown in double-logarithmic scale [after ref. 109].
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Figure 31. DC I–V hysteresis characteristics of Pt/PZT/PCMO(∼60 nm)/Pt layered structure. V-sweep range of 0→+2 Volt→0→−2 Volt→0 (dashed line) corresponds to electroforming. Inset (a) is a schematic configuration of device and measurement setup, inset (b) shows the probing of resistance states in the positive voltage region [after ref. 110].
sweep cycle of positive or negative polarity demonstrated that a large positive voltage sweep cycle (>2.5 Volt) induces a LRS in the positive voltage region as is probed by the repeated read-out at 2 Volt (inset (b) of Fig. 31), and a successive application of a large negative voltage sweep cycle (<−2.5 Volt) induces a HRS as is probed as well in the positive voltage region by the repeated read-out at+2 Volt (inset (b) of Fig. 31).
Similarly, the resistance switching states can also be recognized in the negative voltage region and probed by a negative read bias. Consequently, the hetero-structured PZT/PCMO device showed a bistable resistance switching properties controlled by bipolar switching mode.
Figure 32 presents the obtained I–V hysteresis characteristic for such mesa-structured device with a thinner PCMO layer thickness of∼20 nm that may generate less stress in PZT film. No electroforming was observed in the first voltage sweep, but two clear humps of different sizes observed in the both regions, during changing the bias polarity. The humps are disappeared when the voltage sweep was limited in a small voltage bias range (inset (a) of Fig. 32). According to Bourim et al., this humps are due to the switching polarization current due to the easy ferroelectric dipoles switching within less constrained PZT in the mesastructure, and they can take place only when the ferroelectric dipoles are fully reversed under a larger voltage sweep (>± 2.5 Volt). In addition, switching characteristics in bias pulse mode with alternating serial pulses of positive and negative polarity (inset (b) of Fig. 32) established that both the HRS and LRS arrived at a steady value immediately after reversing the pulse polarity. Also, good endurance was observed under alternating pulses of opposite polarity, as shown in inset (c) of Fig. 32.
3.3.2. Resistive switching effect in graphene oxide/Pr0.7Ca0.3MnO3 films. Recently Kim et al. reported the grephene oxide (GO)/Pr0.7Ca0.3MnO3based resistive switching memory device prepared by a spin-coating method at a low temperature (<300◦C) [107]. In this case GO is used as an insulating layer and conducting PCMO acts as an oxygen exchange layer, for its higher oxygen vacancy concentration at the surface region [107, 144–146]. The Pt/GO/PCMO/Pt device was fabricated on a Pt/Ti/SiO2/Si substrate. 25 nm thick PCMO film was deposited using a spin-coating method using Mn, Ca, and Pr acetate hydrate
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Figure 32. DC I–V hysteresis characteristics of the mesastructured Pt/PZT/PCMO(∼20 nm)/Pt device for 10 consecutive voltage sweeps cycles of 0→+4 Volt→0→−4 Volt→0. Arrows (dashed lines) indicate the sweeping directions and the current evolution. Inset (a) shows the I–V evolution for 10 consecutive voltage sweeps in a short voltage sweep range of 0→+1.5 Volt→0→−1.5 Volt→0.
Insets (b) and (c) show switching characteristics in voltage pulse mode (Pulse amplitude:±3 Volt, pulse width: 100 ms, pulse period: 200 ms and read voltage at+1 Volt) [after ref. 110].
as the source material and 2-methoxyethanol, acetic acid as the solvent. The 30-nm thick graphene oxide was spin-coated on the PCMO layer using a solution containing grapheme oxide particles with H2O solvent. The film was annealed at 150◦C for 30 min using rapid thermal annealing (RTA) process. Finally, Pt top electrodes were deposited by electron beam evaporation.
Figure 33(a) shows the I–V curve for the Pt/GO/PCMO/Pt device, sweep at+1.3 Volt to−1.3 Volt. In this case no forming process was necessary to switch on the device. The set/ reset voltage of the device are∼ − 0.75 Volt and 0.6 Volt, respectively. In contrast, the Pt/PCMO/Pt control sample showed no resistive switching behavior. Mostly PCMO is
Figure 33. (a) Typical I–V hysteresis curves of the GO/PCMO and PCMO device. The inset shows I–V hysteresis curve of the Pt/GO/Pt device. (b) I–V plot of HRS and LRS of the GO/PCMO device in Log scale [after ref. 107].
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as shown in Fig. 33(b). At low voltage (<Von ≈ 0.35 Volt) the I–V curve shows Ohmic behavior in the negative sweep region, because the density of thermally generated free carriers inside the films is predominant over the injected charge carriers. The slope (S) increases to 2, indicating trap-associated space charge limited current (SCLC) theory. When applied voltage reaches the threshold voltage (Vth), the current increases rapidly due to trap-filled condition. The logI–logV plot exhibits linear Ohmic conduction, followed by a SCLC conduction, which corresponds to the Child’s law region [107, 148]. Very stable retention up to 104second was observed in this Pt/GO/PCMO/Pt device at 85◦C without any degradation.
Compared to the other graphene oxide based memory [107, 145, 146], they obtained the
Figure 34. Hysteretic I–V characteristics of a SRO/Nb:STO junction measured with different spans of voltage scan. Bias voltage was swept as (a)−1.4 V→0 V→Vmax→0 V→−1.4 V with Vmax
varied as 1, 2, 3, and 4 V, and (b) 4 V→0 V→Vmin→0 V→4 V with Vminvaried as−0.8, −1.0,
−1.2, and −1.4 V. In both cases, chronological sequence of I–V curves is represented from (1) to (4) [after ref. 104].
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annealed at 400◦C for 30 minute in oxygen ambient at high pressure.
A large hysteresis in I–V characteristics with distinct HRS and LRS are observe at low forward and reverse bias voltage, as shown in Fig. 34(a). As can be seen from the figure, the junction switched to LRS by applying−1.4 Volt bias scan and the junction turned to HRS again by applying 4 Volt bias. Two resistance states were kept unchanged when voltage polarity is changed through 0 Volt [(1)→(2)→(3)→(4)]. I–V curve (4) at HRS during set process is nearly a straight line, agreeing with a Schottky barrier model.
The junction gives leaky I–V characteristics during reset process at positive voltage di-rection. To understand the evolution of hysteresis by widening the voltage scan span, in Fig. 34(a), the bias voltage was swept as−1.4 Volt→ 0 Volt→Vmax→0 Volt→−1.4
Figure 35. (a) I–V switching characteristic of a Fe-doped SrTiO3thin film. Two types of switching behaviour exhibiting opposite polarities can be seen in one and at the same junction. (b) Conductive AFM topography and current image of a junction after electroforming and top electrode removal.
A well conducting crater (green) as well as a medium conducting region around the crater (orange) is shown [after ref. 105].
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age pulses, the resistance states was switched between rather steady HRS and variable LRS.